1. Field of the Invention
The present invention relates to a temperature control method for a magnetic resonance system, and more particularly to a temperature control method for magnetic field components of a permanent magnet arrangement of a magnetic resonance system.
2. Description of the Prior Art
The magnetic field component used to produce and regulate the magnetic field, including magnetic blocks, pole plates, magnet laminations and shim irons, is the core of a permanent magnetic resonance system. Referring to FIG. 1, in a typical permanent magnetic resonance system, magnetic blocks 20a, 20b, pole plates 30a, 30b, magnet laminations 40a, 40b and shim irons 50a, 50b are oppositely arranged in order on the lower and upper magnetic yokes 10a, 10b. 
Stability is an important parameter of the magnetic field components. There are many external conditions that affect the stability of a permanent magnet and the most important one is the temperature stability. However the magnetic field components of a permanent magnet of a magnetic resonance system, in particular magnetic blocks 20a, 20b used to produce a magnetic field, are very sensitive to temperature change. The reason is that the characteristic of permanent magnet material changes notably with temperature, and thus the magnetic field excited by the magnet will change as well, which is reflected in fluctuation of magnetic field strength and decreased homogeneity. Such a change of the magnetic field will directly lead to decreased imaging quality of the magnetic resonance system.
To maintain the stability of the field strength, it is conventional to use insulating materials to cover the lower and upper magnetic yokes 10a, 10b and the magnetic field components, or to use a cooling or heating device either on or between the lower and upper magnetic yokes 10a, 10b to keep the temperature of the magnetic field components constant.
The method of using insulating materials to cover the lower and upper magnetic yokes 10a, 10b and magnetic field components not only increases the sizes of these components, but also does not achieve satisfactory results. This is because the insulating materials reduce the sensitivity of the magnetic field components to the surrounding temperature change, so they augment the effect of some heat producing parts in the magnetic resonance system, such as gradient coils 60a, 60b mounted on shim irons 50a, 50b as shown in FIG. 1, on the temperature stability of the magnetic field components.
The method of using a cooling device on the lower and upper magnetic yokes 10a, 10b makes it necessary to wrap the electronic cooling device around the entire length of the lower and upper magnetic yokes 10a, 10b to cool the magnetic field components down to within a temperature range of 10 to 50 degrees lower than the surrounding temperature, and to use insulating materials to cover the lower and upper magnetic yokes 10a, 10b to reduce the effect of the surrounding temperature change. The above cooling device method can achieve a better field stability result but the oversized structures and high power consumption greatly reduce its practicability.
Use of a heating device on or between the lower and upper magnetic yokes 10a, 10b is relatively simple in structure, as shown in FIG. 1. The heating elements 100a, 100b are fitted inside (or on the interior surface of) the lower and upper magnetic yokes 10a, 10b to control the temperature of the magnetic field components. Further, the heating elements 100a, 100b are connected to the temperature control units 70a, 70b, respectively, to control the temperature on a real-time basis. However, such a single-channel heating device is only capable of temperature control at a narrow range. When closed to the magnetic blocks 20a, 20b, the heating device is able to keep the temperature of the magnetic blocks 20a, 20b constant. However, the heating device no longer has effective control over the temperature of the magnetic blocks 20a, 20b at a certain distance away, and of those farther away, pole plates 30a, 30b, magnetic laminations 40a, 40b and shim irons 50a, 50b, and reacts quite slowly to the temperature change in these areas as well.
Chinese application No. 99800973.3 discloses an improvement over the above heating device and provides a multi-channel heating device. The multi-channel heating device provides heating elements within the lower and upper magnetic yokes 10a, 10b, magnetic blocks 20a, 20b and pole plates 30a, 30b to effectively keep the magnetic field components in a constant temperature state so that a stable field strength is obtained.
The constant temperature state of the magnetic field component achieved by using multi-channel heating devices, however, is only a static stable state by providing a heating element at both sides of the magnetic blocks 20a, 20b, for example, so as to force the magnetic blocks 20a, 20b into a static constant temperature state. If the static stable state is broken, However, for example if a thermal disturbance (sudden temperature change) occurs at the magnetic blocks 20a, 20b, and since both sides are forced into a static constant temperature state by the heating devices, the thermal disturbance will have a dramatic, concussive impact on the internal temperature stability of the magnetic blocks 20a, 20b. Such impact will have a severe effect on the size and homogeneity of the field strength and a longer time is needed to offset the impact.